Maybe I can use a Illuminometer to confirm the lux?
You could, but what really will it tell you? Light measurement sounds simple, but gets really complicated, really fast. You end up realizing that it's only worthwhile if you can actually do a spectral output plot across the entire range of wavelengths the light source emits.
Keep in mind that if you try to measure the lux output of a narrow-bandwidth light source (e.g. an electroluminescent panel), you get wildly skewed results because the lux scale is a weighted scale based on a bell-curve that roughly matches human spectral sensitivity. For example, look at a plot of the scotopic and photopic luminosity curves (a lux measurement will be based on either; the measurement instrument should have this info in its specifications somewhere), and two theoretical single-wavelength light sources of 450nm blue and 540nm green. Assume that both light sources have the same electrical efficiency; i.e. they generate the same number of photons per amount of energy put into them:
Note how the blue light source will register virtually nothing on the lux scale if based on the photopic curve or substantially more of the measurement is based on a scotopic response. For the green source, a similar pattern is visible, but more importantly, it registers a far stronger signal than the blue source. So the way the measurement is weighted is quite relevant as this skews the results massively.
A further concern is that I argued that both theoretical light sources have the same electrical efficiency. But this in itself is actually inconclusive in itself, if you think about it. After all, blue photons have a shorter wavelength and thus are higher-energy photons than green ones. So even if both light sources emit the same number of photons, the amount of energy they emit in the form of light still differs. So which would you want to measure - number of photons, or amount of energy?
Finally, there's the question of the observer; we touched upon it above when discussing lux/lumens, and the story changes yet again if you take one particular film or paper and consider its spectral sensitivity. What will be the benchmark for your measurement?
So by all means, measure the lumens your sensitometer puts out, but the problem is that the number you will get will be very difficult to interpret, and it will most likely be meaningless from a practical viewpoint.
So latching back to something mentioned earlier in the thread, I think by
@ic-racer : that the USAF relies/relied on
comparative measurements only. I think there's very good reason for this, and it's mostly because an
absolute measurement will be very difficult to establish and it's doubtful it adds much value in the real world.